Combustion chamber screech eliminator



1960 w. E. KASKAN ETAL 2,919,550

COMBUSTION CHAMBER SCREEICH ELIMINATOR Filed April 13, 1955 INVENTORS. are i. A/flEA-"EA/ BY W417 (Ail I United States Patent COMBUSTION CHAMBER SCREECH ELllVIINATOR Application April 13, 1955, Serial No. 501,033

' 2 Claims. (Cl. 60-39.72)

1 This invention relates to combustion chambers and,

more particularly, to a screech eliminator for a combustion chamber. p

In certain combustion chambers, the former practice has been to place an object such as a flameholder in the path of the combustible fluid flowing through the combustion chamber which may be an afterburner of a jet engine." The flameholder reduces the velocity of the combustible fluid in the afterburner sufficiently to allow combustion to proceed to completion in the afterburner. In the absence of a flameholder, the velocity of the combustible fluid would prevent flame propagation within the afterburner since its velocity, generally, is greater than the velocity of flame propagation.

It has been found, however,.that the use of a flameholder causes low frequency and high frequency burner oscillations within the afterburner. The high frequency oscillations are called screech. The condition known as screech is undesirable for two reasons. First of all, the screech is loud and piercing and presents a noise problem in that the sound is loud enough to be a nuisance. Secondly, when screech occurs, the burner is in great danger of undergoing mechanical failure due to the destructively high oscillation pressures and increasing wall temperatures within the afterburner which are a result of the screech phenomenon.

The cause of screech has hitherto been unknown. It has been found by experiment, however, that under certain conditions of operation there is a change of flame area in the afterburner which is periodic with respect to time. The change in flame area which is periodic with respect to time has been isolated as the source of screech. The cause of the periodic change in flame area has been found to be the periodic formation of vortices at the trailing edges of the flameholder and periodic shedding of these vortices from the flameholder trailing edges due to the high velocity of the combustible fluid.

Accordingly, it is the object of this invention to provide a device which will prevent the formation of vortices at the trailing edges of a flameholder thereby eliminating low frequency oscillations as well as the high frequency oscillations known as screech.

Briefly stated, in accordance with one aspect of this invention, a screech eliminator member is provided near the trailing edge of the flameholder in order to prevent the formation of vortices and thereby prevent the occurrence of low frequency and high frequency oscillations known as screech.

The invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.-

In the drawing, Figure 1 is a longitudinal view of an afterburner, partly in section, showing the eliminator mounted therein; Figure 2 is a sectional view of the apparatus of Figure 1 along the line 22; Figure 3 is an Patented Jan. 5, 1960 enlarged fragmentary sectional view of the flameholder and screech eliminator.

Referring to Figures 1 and 2, the structure of the device comprises a combustion chamber which may be in the form of a jet engine afterburner 1, having a variable discharge nozzle 2. The area of the variable discharge nozzle 2 is varied by means of the deflector 3 movable above the pivot 4. An inner cone 5 is mounted centrally of the .afterburner 1 and a substantially V-shaped flameholder 6 is secured to the inner cone 5 by means of the flameholder supports 7. Although the flameholder 6 is shown to be substantially V shaped, it is not limited to this shape and it may take any shape satisfactory for holding a flame within the combustion chamber or afterburner 1.

' The screech eliminators 8 are mounted within the afterburner 1 by means of the outer supports 9 secured to the afterburner 1 and the inner supports 10 attached to the inner cone 5; The screech eliminators 8 are mounted near the trailing edges 11 of the flameholder 6. It has been found experimentally that the screech eliminators 8 operate most efficiently when placed in close proximity to an imaginary surface passing through the trailing edge 11 of theflameholder 6 and parallel to the direction of flow of the combustible fluid. The reasons for so placing the screech eliminators 8 will become evident upon reading the description of the operation hereinafter set forth; Referring to the drawings it will be seen that our screech eliminators, as shown, comprise a pair of concentric ring members each having a cross-sectional configuration which is relatively long and narrow with respect to the direction of flow in the afterburner 1. Although the outer contour of each ring member is not limited to that shown on the drawing, it will be understood by those skilled in the art that the most desirable outer contour will be one which offers a minimum resistance to fluid flow such as the streamlined shape shown in the drawing.

During operation, the combustible fluid flows rearwardly toward the discharge nozzle 2 in the afterburner 1 as shown by the arrow 12 and a continuously burning flame is held at or near the trailing edges 11 of the flameholder 6. Due to the velocity of the fluid and the sudden change in the cross-sectional area of the afterburner as the fluid passes the trailing edges 11 of the flameholder 6, there is a tendency to form vortices at each trailing edge 11 of the flameholder 6. Since, as aforesaid, the formation of such vortices may ultimately result in low frequency as well as high frequency oscillations known as screech if they are periodically shed from the trailing edge 11 of the flameholder 6, the screech eliminators 8 are mounted immediately to the rear of the trailing edges 11 of the flameholder 6 and in close proximity to an imaginary surface passing through the trailing edges 11 of the flameholder 6 and parallel to the direction of flow of the combustible fluid. By so placing the screech eliminators 8, a flow pattern as shown in Figure 3, is obtained which prevents the formation of vortices at the trailing edges 11 of the flameholder 6 and which thereby eliminates the cause of low frequency as well as high frequency oscillations known as screech within the afterburner 1.

The screech eliminators 8 will continue to operate etfectively even if placed in a position somewhat inwardly or outwardly with respect to the center of the afterburner 1 from the exact position shown in Figure 3 of the drawing. Any change from the position shown will be effective if the screech eliminators 8 remain in such a position that they will prevent the formation of vortices at the trailing edge 11 of the flameholder 6. The position of the screech eliminators 8 in Figure 3 is more desirable than a position somewhat closer to the imaginary cylinder passing through the trailing edge 11 of theflameholder 6. In the position shown, the eliminators 8 not only prevent the formation of vortices but a percentage of unignited combustible fluid passes over the surfaces 13 and 14 from the trailing edge 11 of the flameholder 6 cooling the eliminators 8 and prolonging their useful life.

While a particular embodiment of the invention has been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention, and itis intended to cover in the appended claims all: such changes and modifications that come. within: the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

- 1. In a combustion chamber defining a flow path for a combustible fluid, a flameholder having at least one trailing edge mounted in the flow path of the combustible fluid, said trailing edge tending to form Screech-causing vortices of combustible fluid as said fluid passes over the trailing edge, and screech elimination means mounted adjacent to and immediately downstream of the flameholder trailing edge, said Screech-elimination means com prising at least one annular member having a long and narrow cross-sectional configuration to offer a minimum resistance to fluid flow, the longitudinal axis of said configuration being parallel to the direction of fluid flow.

2. In a combustion chamber defining a flow path for a combustible fluid, the combination of an annular flameholder mounted in said chamber and having two concentric trailing edges positioned in the flow path of the combustible fluid, and screech-elimination means comprising two concentric ring members mounted adjacent to and immediately downstream of said flameholder, said ring members being relatively long and narrow in crosssection in the direction of the fluid flow and having a streamlined outer contour offering the least resistance to said fluid flow, said ring members being positioned in close proximity to imaginary surfaces passing through each of said trailing edges and parallel to the direction of flow, said ring members acting to prevent the periodic build-up of vortices of combustible fluid on the flameholder trailing edges to eliminate screech-causing high frequency oscillations common to said periodic build-up of the vortices.

References Cited in the file of this patent UNITED STATES PATENTS 1,684,567 Wragg Sept. 18, 1928 1,779,026 Wragg Oct. 21, 1930 2,448,966 Fales Sept. 7, 1948 2,659,195 Bolanovich Nov. 17, 1953 2,692,480 Viaud et al Oct. 26, 1954 FOREIGN PATENTS 1,085,458 France July 28, 1954 (Corresponding to Great Britain, No. 761,167, Nov. 14, 1956) OTHER REFERENCES Third Symposium on Combustion Flame and Explosion Phenomena, pages 48-49, by The Standing Committee on Combustion Symposia, published by The Williams and Wilkins Company, Baltimore, Md., 1949. 

